1. Field of the Invention
The present invention is directed to combustible gas analysis apparatus. More specifically, the present invention is directed to a gas analysis apparatus for determining the calorific content of combustible gases.
2. Description of the Prior Art
The well-known Wobbe Index of a combustible gas is defined as the amount of heat released by a burner of constant orifice, McGraw-Hill Dictionary of Science, 1979 and stated mathematically by the following relationship: ##EQU1## where H is the caloric value of the gas per unit volume, e.g., BTU/ft.sup.3, and SG is the specific gravity of the gas. The Wobbe Index is a quantity used in heating technology since different combinations of gases supplied to a gas heated apparatus under the same pressure provide equal heat production so that the apparatus need not be readjusted as long as the Wobbe indices are the same. If, for example, a mixture of gases from different sources is burned in an industrial operation, the gases must be mixed in such a proportion so that a gas is obtained having a constant Wobbe Index. One prior art method of determining the Wobbe Index of a combustion gas involves the combination of a calorimeter, a density meter and a computing circuit, e.g., a microprocessor. These parts while they may be combined into a single instrument produce an overall device which is very costly and exhibits sluggish operation whereby rapid changes in the gas mixture Wobbe Index are measured at a slow rate and the resulting correction, if necessary, is also applied at a correspondingly slow rate. It has been discovered that when gas mixtures having different compositions and different Wobbe Indices are burned with equal quantities of air, the oxygen content of the exhaust gas shows a direct correlation with the Wobbe Index. Accordingly, for purposes of measurement and control, it is not necessary to measure the Wobbe Index as such and it is sufficient to measure only the oxygen content in the exhaust gas. One prior art apparatus for producing this type of operation includes a sampling line containing a flow control nozzle for withdrawing a gas sample, a means for adjusting the gas sample so that the pressure difference through the control nozzle has an adjustable constant value, a means for feeding a constant volume of air as a source of combustion oxygen into the gas stream sample, a combustion chamber, a burner in the combustion chamber to completely burn the gas-air mixture, an outlet for the burned gas from the combustion chamber and an oxygen sensor in the combustion chamber to sense the oxygen quantity of the exhaust gas. This oxygen content is a quantity which is correlated to the Wobbe Index of the measured gas. However, by actually measuring or providing a quantity which represents the Wobbe Index the thermal delivery rate, i.e., BTU/minute, can be obtained to control the industrial heating operation. On the other hand, since the Wobbe Index is only related to the actual BTU content of the combustible gas, the actual measurement of the calorific, e.g., BTU, content of the combustible gas would provide a quantity which is even more directly applicable to the control of industrial heating operation. Such a measurement would provide a more efficient means of control to a furnace requiring a constant BTU per minute input without regard to changes in gas supply composition, density and BTU content of the combustible gas.